Power cable with a thick insulation layer and a method for its manufacture

ABSTRACT

A power cable comprising a: (A) Conductor, (B) First semiconductor in contact with the conductor; (C) First insulation layer in contact with the first semiconductor; (D) Second semiconductor layer in contact with the first insulation layer; (E) Third semiconductor layer in contact with the second semiconductor layer; (F) Second insulation layer in contact with the third semiconductor layer; and (G) Fourth semiconductor layer in contact with the second insulation layer.

FIELD OF THE INVENTION

This invention relates to power cables. In one aspect the inventionrelates to power cables comprising a thick insulation layer while inanother aspect, the invention relates to a method of manufacturing apower cable with a thick insulation layer.

BACKGROUND OF THE INVENTION

Extruded high-voltage cables utilized thick layers of insulation toensure reliable service life. Such cable designs lead to challenges incable manufacturing including the following: i) sufficiently longvulcanization processes to ensure adequate crosslinking of the innerlayers of the insulation, ii) sufficient cooling process to cool thecable to enable reeling, iii) controlled cooling to minimizelongitudinal stresses leading to “shrink-back” of the conductor from theinsulating layers, iv) difficulty in cable centering in somemanufacturing configurations in which heavy-wall cable designs aresubjected to gravitational forces that lead to sag of the molteninsulation around the conductor, v) long degassing times required toremove crosslinking byproducts via a diffusion process through thicklayers of insulation, and vi) limited availability of cable linessuitable for high-voltage cable manufacturing.

The state-of-the-art cable manufacturing process involves a true-tripleextrusion of an insulation layer between two semiconductor (shielding)layers in a concentric fashion around the conductor. This processprovides smooth interfaces between the insulation and the surroundingmaterials and avoids the introduction of contamination in amultiple-step process. However, for thick insulation layers thermallyinduced crosslinking and subsequent evacuation or degassing ofcrosslinking byproducts leads to low productivity.

Although the potential for extruding the insulation layer in multiplesteps is known, no reference suggests a multiple extruded insulationlayer that is separated by an intermediate layer of high conductivityand/or permittivity. The multiplicity of layers in the absence of suchan intermediate layer leaves open the potential for introduction ofcontamination or voids between the mating insulation layers, which wouldnegatively impact cable reliability. Moreover, since extruding theinsulation layer in multiple steps would require the steps to beperformed in relatively rapid succession so that the secondsemiconductor layer can be applied to allow collection and storage ofthe cable, the advantage of reduced crosslinking and degassing times dueto the curing of thinner layers is lost since the insulation layer isjust as thick as if it had been extruded in a single step. Stillfurther, use of multiple extrusion layers in a single pass process wouldrequire existing manufacturing processes to install an additionalextruder in an attempt to achieve the quality that is currently achievedin the state-of-the-art true-triple process.

SUMMARY OF THE INVENTION

In one embodiment the invention is a method of manufacturing a powercable comprising a conductor, semiconductor layers and insulationlayers, the process comprising the steps of:

-   -   (A) Extruding about the conductor a first insulation layer        positioned between first and second semiconductor layers to make        an inner power cable comprising:        -   (1) The conductor which is in contact with,        -   (2) A first semiconductor layer which is also in contact            with,        -   (3) A first insulation layer which is also in contact with,        -   (4) A second semiconductor layer, and    -   (B) Extruding about the inner power cable a second insulation        layer positioned between third and fourth semiconductor layers        to make the power cable comprising the inner power cable of        which the second semiconductor layer is in contact with:        -   (5) The third semiconductor layer which is also in contact            with,        -   (6) The second insulation layer which is also in contact            with,        -   (7) The fourth semiconductor layer.

In one embodiment the invention is a power cable comprising a:

-   -   (A) Conductor having an exterior facial surface,    -   (B) First semiconductor layer having first and second facial        surfaces, the first facial surface of the first semiconductor        layer in contact with the exterior facial surface of the        conductor;    -   (C) First insulation layer having first and second facial        surfaces, the first facial surface of the first insulation layer        in contact with the second facial surface of the first        semiconductor;    -   (D) Second semiconductor layer having first and second facial        surfaces, the first facial surface of the second semiconductor        layer in contact with the second facial surface of the first        insulation layer;    -   (E) Third semiconductor layer having first and second facial        surfaces, the first facial surface of the third semiconductor        layer in contact with the second facial surface of the second        semiconductor layer;    -   (F) Second insulation layer having first and second facial        surfaces, the first facial surface of the second insulation        layer in contact with the second facial surface of the third        semiconductor layer; and    -   (G) Fourth semiconductor layer having first and second facial        surfaces, the first facial surface of the fourth semiconductor        layer in contact with the second facial surface of the second        insulation layer.

In one embodiment both the first and second passes are tripleextrusions. The method of this invention allows for both passes to beconducted with the same equipment, and maintains good qualityinterfaces. The intermediate (second and third) semiconductor layersprovide a barrier of high conductivity or permittivity, and theyencapsulate any potential contamination in a manner in which stressconcentration can be avoided. In other words, any contamination that mayaccumulate on the second semiconductive layer is trapped between thesecond and third semiconductive layers when the third semiconductivelayer is applied to the second semiconductive layer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of one embodiment of the process formaking an inner power cable of this invention.

FIG. 2 is a schematic illustration of one embodiment of an inner powercable of this invention.

FIG. 3 is a schematic illustration of one embodiment of extruding thethird and fourth semiconductor and second insulation layers over aninner power cable.

FIG. 4 is a schematic illustration of one embodiment of a power cable ofthis invention.

FIG. 5 is a schematic illustration of the component parts of a powercable of this invention in an exploded format.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Definitions

Unless stated to the contrary, implicit from the context, or customaryin the art, all parts and percents are based on weight and all testmethods are current as of the filing date of this disclosure. Forpurposes of United States patent practice, the contents of anyreferenced patent, patent application or publication are incorporated byreference in their entirety (or its equivalent US version is soincorporated by reference) especially with respect to the disclosure ofdefinitions (to the extent not inconsistent with any definitionsspecifically provided in this disclosure) and general knowledge in theart.

The numerical ranges in this disclosure are approximate, and thus mayinclude values outside of the range unless otherwise indicated.Numerical ranges include all values from and including the lower and theupper values, in increments of one unit, provided that there is aseparation of at least two units between any lower value and any highervalue. As an example, if a compositional, physical or other property,such as, for example, temperature, is from 100 to 1,000, then allindividual values, such as 100, 101, 102, etc., and sub ranges, such as100 to 144, 155 to 170, 197 to 200, etc., are expressly enumerated. Forranges containing values which are less than one or containingfractional numbers greater than one (e.g., 1.1, 1.5, etc.), one unit isconsidered to be 0.0001, 0.001, 0.01 or 0.1, as appropriate. For rangescontaining single digit numbers less than ten (e.g., 1 to 5), one unitis typically considered to be 0.1. These are only examples of what isspecifically intended, and all possible combinations of numerical valuesbetween the lowest value and the highest value enumerated, are to beconsidered to be expressly stated in this disclosure. Numerical rangesare provided within this disclosure for, among other things, thethickness of the various power cable layers.

“Comprising”, “including”, “having” and like terms mean that thecomposition, process, etc. is not limited to the components, steps, etc.disclosed, but rather can include other, undisclosed components, steps,etc. In contrast, the term “consisting essentially of” excludes from thescope of any composition, process, etc. any other component, step etc.excepting those that are not essential to the performance, operabilityor the like of the composition, process, etc. The term “consisting of”excludes from a composition, process, etc., any component, step, etc.not specifically disclosed. The term “or”, unless stated otherwise,refers to the disclosed members individually as well as in anycombination.

“Cable”, “power cable” and like terms means at least one conductive wireor optical fiber within a protective jacket or sheath. Typically a cableis two or more wires or optical fibers bound together, typically in acommon protective jacket or sheath. The individual wires or fibers maybe bare or covered. The protective jacket or sheath can comprise one ormore semiconductor layers, and/or insulation layers, and/or metallictapes, and/or exterior coatings. Combination cables may contain bothelectrical wires and optical fibers. The cable, etc., can be designedfor low, medium, high or extra-high voltage applications. Typical cabledesigns are illustrated in U.S. Pat. Nos. 5,246,783, 6,496,629 and6,714,707.

“Facial surface”, “planar surface”, “top surface”, “bottom surface” andthe like are used in distinction to “edge surface”. If rectangular inshape or configuration, an article, e.g., a sheet or film, will comprisetwo opposing facial surfaces joined by four edge surfaces (two opposingpairs of edge surfaces, each pair intersecting the other pair at rightangles). If circular in configuration, then the article will comprisetwo opposing facial surfaces joined by one continuous edge surface. Inthe context of a cable, the layers are cylindrical in shape and as such,the inner and outer, or first and second, facial surfaces are curved.

“Layer” means a single thickness, coating or stratum spread out orcovering a surface.

“Multi-layer” means two or more layers with adjacent layers in contactwith each other.

Conductor

The conductor is the core of the cable. It is the component of the cableabout which the first semiconductor layer is in wrapped and in contact,and it can comprise a single, electrically conducting wire or a bundleof electrically conducting wires. These wires are typically metal,preferably copper or aluminum. In power transmission aluminumconductor/steel reinforcement (ACSR) cable, aluminum conductor/aluminumreinforcement (ACAR) cable, or aluminum cable is typical. If theconductor comprises a bundle of wires, the individual wires can be boundtogether with any suitable binding or potting material, e.g., an epoxyresin.

In one embodiment the conductor comprises an optical fiber.

Chemical Composition of the Semiconductor and Insulation Layers

The composition of the semiconductor and insulation layers are notcritical to the invention and thus can vary widely and be made fromvirtually any polymer, most typically a crosslinkable, thermoplasticpolymer. These polymers are well known in the art and in someembodiments, the semiconductor and insulation layers are made from thesame polymer with the layers differing only by the presence or absenceof conducting filler, e.g., conductive carbon black, metal particulate,and the like. The polymers can be crosslinked in any convenient manner,but are typically peroxide and/or moisture cured.

Nonlimiting examples of suitable polymers include styrenic blockcopolymers (e.g., SEBS), ethylene-based elastomers/plastomers (e.g.,ENGAGE™ and AFFINITY ethylene-based copolymers), ethylene blockcopolymers (OBCs) (e.g., INFUSE™ 9507 or 9100 OBC) and propylene-basedplastomers and elastomers (e.g. VERSIFY™ 3300 and 4200). Other TPEpolymers useful in the practice of this invention include, for example,but are not limited to, thermoplastic urethane (TPU), ethylene/vinylacetate (EVA) copolymers (e.g., ELVAX 40L-03 (40% VA, 3MI) (DuPont)),ethylene/ethyl acrylate (EEA) copolymers (e.g., AMPLIFY) and ethyleneacrylic acid (EAA) copolymers (e.g., PRIMACOR) (The Dow ChemicalCompany), polyvinylchloride (PVC), epoxy resins, styrene acrylonitrile(SAN) rubber, and Noryl® modified PPE resin (amorphous blend ofpolyphenylene oxide (PPO) and polystyrene (PS) by SABIC), among others.Also useful are olefinic elastomers including, for example, very lowdensity polyethylene (VLDPE) (e.g., FLEXOMER® ethylene/1-hexenepolyethylene, The Dow Chemical Company), homogeneously branched, linearethylene/a-olefin copolymers (e.g. TAFMER® by Mitsui PetrochemicalsCompany Limited and EXACT® by DEXPlastomers), and homogeneouslybranched, substantially linear ethylene/a-olefin polymers (e.g.,AFFINITY® ethylene-octene plastomers (e.g., EG8200 (PE)) and ENGAGE®polyolefin elastomers, The Dow Chemical Company). Substantially linearethylene copolymers are more fully described in U.S. Pat. Nos.5,272,236, 5,278,272 and 5,986,028.

Additional olefinic interpolymers useful in the present inventioninclude heterogeneously branched ethylene-based interpolymers including,but are not limited to, linear medium density polyethylene (LMDPE),linear low density polyethylene (LLDPE), and ultra low densitypolyethylene (ULDPE). Commercial polymers include DOWLEX™ polymers,ATTANE™ polymer, FLEXOMER™, HPDE 3364 and HPDE 8007 polymers (The DowChemical Company), ESCORENE™ and EXCEED™ polymers (Exxon MobilChemical). Nonlimiting examples of suitable TPUs include PELLETHANE™elastomers (Lubrizol Corp. (e.g., TPU 2103-90A); ESTANE™, TECOFLEX™,CARBOTHANE™, TECOPHILIC™, TECOPLAST™ and TECOTHANE™ (Noveon);ELASTOLLAN™, etc. (BASF), and commercial TPUs available from Bayer,Huntsman, the Lubrizol Corporation and Merquinsa.

The layers may and usually do contain one or more additives includingbut not limited to processing aids, fillers, crosslinking agents,crosslinking co-agents, coupling agents, ultraviolet absorbers orstabilizers, antistatic agents, nucleating agents, slip agents,plasticizers, lubricants, viscosity control agents, tackifiers,anti-blocking agents, surfactants, extender oils, acid scavengers, andmetal deactivators. Additives, other than fillers, are typically used inamounts ranging from 0.01 or less to 10 or more wt % based on the weightof the composition. Fillers are generally added in larger amountsalthough they the amount can range from as low as 0.01 or less to 50 ormore wt % based on the weight of the composition. Examples of fillersinclude but are not limited to clays, precipitated silica and silicates,fumed silica, calcium carbonate, ground minerals, and carbon blacks withtypical arithmetic mean particle sizes larger than 15 nanometers.Conductive additives and fillers, e.g., those that yield a conductivityof less than 1,000 ohms per meter (ohm-m) in a filled composition, aretypically used in the semiconductive layers, and nonconductive or poorlyconductive additives and fillers, e.g., those that yield an insulationvolume resistivity of no less than 10⁸ ohm-m, are typically used in theinsulation layers.

Compounding and Fabrication

Compounding of cable layer material can be effected by standardequipment known to those skilled in the art. Examples of compoundingequipment are internal batch mixers, such as a Banbury™ or Bolling™internal mixer. Alternatively, continuous single, or twin screw, mixerscan be used, such as Farrel™ continuous mixer, a Werner and Pfleiderer™twin screw mixer, or a Buss™ kneading continuous extruder.

A cable containing semiconductor and insulation layers can be preparedwith various types of extruders, e.g., single or twin screw types. Adescription of a conventional extruder can be found in U.S. Pat. No.4,857,600. An example of co-extrusion and an extruder therefore can befound in U.S. Pat. No. 5,575,965. A typical extruder has a hopper at itsupstream end and a die at its downstream end. The hopper feeds into abarrel, which contains a screw. At the downstream end, between the endof the screw and the die, there is a screen pack and a breaker plate.The screw portion of the extruder is considered to be divided up intothree sections, the feed section, the compression section, and themetering section, and two zones, the back heat zone and the front heatzone, the sections and zones running from upstream to downstream. In thealternative, there can be multiple heating zones (more than two) alongthe axis running from upstream to downstream. If it has more than onebarrel, the barrels are connected in series. The length to diameterratio of each barrel is in the range of about 15:1 to about 30:1. Inwire coating where a layer is crosslinked after extrusion, the cableoften passes immediately into a heated vulcanization zone downstream ofthe extrusion die. The heated cure zone can be maintained at atemperature in the range of about 200° C. to about 350° C., preferablyin the range of about 170° C. to about 250° C. The heated zone can beheated by pressurized steam, or inductively heated pressurized nitrogengas.

Method of Manufacture

FIG. 1 describes one embodiment of the manufacture of inner power cable10 (FIG. 2). Conductor 11 is fed to triple extrusion die 12 in which thefirst semiconductor layer, first insulation layer and secondsemiconductor layer are applied in concentric fashion to it. Tripleextrusion is a known process, and in it each layer is appliedsimultaneously, or near simultaneously, such that the firstsemiconductive layer is applied over and in contact with the conductor,the first insulation layer is applied over and in contact with the firstsemiconductive layer, and the second semiconductive layer is appliedover and in contact with the first insulation layer. The firstsemiconductive layer is fed to die 12 from extruder 13, the firstinsulation layer is fed to die 12 from extruder 14, and the secondsemiconductive layer is fed to die 12 from extruder 15.

Inner power cable 10 is passed through continuous vulcanization (CV)tube 16 in which the various layers are crosslinked (partially or fullydepending upon the composition of the individual layer), through coolingstation 17, and eventually to take-up reel 18. Depending upon theconductor, chemical composition of the various layers and the physicalconstruction of the cable (e.g., layer thicknesses), in some embodimentsthis inner power cable can serve as a low or medium voltage cable. Asseen in FIG. 2, inner power cable 10 comprises conductor 11 (here shownas a bundle of wires), first semiconductor layer 13A, first insulationlayer 14A and second semiconductor layer 15A. The layers are arrangedabout conductor 11 in concentric circles.

In another embodiment inner power cable is made by first extruding thefirst semiconductor layer over the conductor, then extruding the firstinsulation layer over the first semiconductor layer, and then extrudingthe second semiconductor layer over the first semiconductor layer. Insome embodiments each layer is at least partially cured before the nextlayer is applied. In one embodiment the first semiconductive and firstinsulation layers are applied simultaneously or near simultaneouslybefore the second semiconductor layer is applied. In one embodiment thefirst insulation and second semiconductor layers are appliedsimultaneously or near simultaneously over the first semiconductorlayer. Triple extrusion is the preferred process of applying the threelayers to the conductor.

The third and fourth semiconductor layers and the second insulationlayer are then applied to the inner power cable in the same manner astheir counterparts were applied to the conductor. Indeed, as shown inFIG. 3 the same equipment that was used to construct inner power cable10 can be used to complete the construction of power cable 20 (FIG. 5).In this instance, inner power cable 10 replaces conductor 11, andextruder 13 feeds die 12 with the composition that becomes thirdsemiconductor layer 13B, extruder 14 feeds die 12 with the compositionthat becomes second insulation 14B, and extruder 15 feeds die 12 withthe composition that becomes fourth semiconductor 15B. Alternatively,the compositions that become layers 13B, 14B and 15B can be appliedindividually or in combination with the composition that becomes anadjacent layer as described above.

FIGS. 4 and 5 show schematically the construction of power cable 20. Thefirst extrusion pass constructs inner power cable 10, and the secondextrusion pass applies second extrusion coatings 19. The combination ofthese two constructions is power cable 20.

The method of this invention allows for the construction of power cableswith thick insulation, i.e., a total thickness of 9 millimeters (mm) ormore. “Total thickness” means the sum of the thicknesses of eachinsulation layer in the cable. The thickness of each insulation layercan be the same or different.

Power cables with a single thick insulation layer are difficult tomanufacture for one or more of the following reasons: (i) they require asufficiently long vulcanization process to ensure adequate crosslinkingof the inner layers of the insulation, (ii) the cable requiressufficient cooling to enable reeling, (iii) controlled cooling isrequired to minimize longitudinal stresses that can lead to shrink-backof the insulating layers from the conductor, (iv) difficulty in cablecentering in some manufacturing configurations in which heavy-wall cabledesigns are subjected to gravitational forces that lead to sag of themolten insulation around the conductor, (v) long degassing timesrequired to remove crosslinking byproducts via diffusion through thethick layer of insulation, and (vi) limited availability of cable linessuitable for high-voltage cable manufacturing. To avoid some of theseproblems, the insulation layer can be applied in multiple passes butthis can lead to the entrapment of contaminants between the insulationlayers.

The process of this invention can produce a power cable with a totalinsulation thickness suitable for high voltage applications and avoidsome or all of these difficulties. This new process and cable designemploy multiple production passes through a co-extrusion andcross-linking production line in which instantaneous line rate can bedramatically increased due to the reduced limitations from curing andcooling. Shrink-back and sag is significantly reduced as the inner powercable on the second pass has already been cooled. Additionally, thismulti-step process may be suitable for high-voltage cable productionusing equipment that is currently utilized for medium-voltage cables.

This new cable design also employs a semiconductor or field gradinglayer between two insulating layers. This intermediatesemiconductive/field grading layer assists in the cable meetingstringent performance measures by encapsulating any contamination whichmay have been introduced on the surface of the second semiconductorlayer during the production of the of the inner power cable. Moreover,this new process and cable design may increase manufacturing processlatitude and free up capacity on large continuous vulcanization (CV)lines through line speed increases (a smaller core can be made on mediumvoltage CV lines). These benefits should also extend into extra-highvoltage cable manufacture.

Another potential benefit of this invention is the ability to utilizedifferent insulation types in the inner and outer insulation layers. Notonly can this mean very high quality (cleanest and most costly)materials only for the inner (highest stress) layer with less costlymaterials in the outer layer, but it would also provide opportunities tointroduce more flexible or filled insulation layers in the outer layeras a means to increase cable flexibility. Of course, although theinvention has been described in terms of two insulation layers separatedby a semiconductor layer, the process can be repeated to produce a powercable with three or more insulation layers, each layer separated by asemiconductor layer. Also, although the invention has been described interms of a power cable, it can be used to manufacture other cable aswell, e.g., low and medium voltage cables, fiber optic cables, etc.

The relative thicknesses of the semiconductor and insulation layers ofthe cables of this invention can vary to convenience, but typically eachsemiconductor layer is narrower or of less thickness than eachinsulation layer. Each semiconductor layer is typically, but notnecessarily, of like thickness, e.g., from 0.2 to 1.5 mm, more typicallyfrom 0.4 to 1 mm, although the second and third semiconductor layers canbe considered to form effectively, if not physically (an interface canexist between the two layers), a merged layer with a combined thicknessof the two individual layers, e.g., from 0.4 to 3 mm. Of course, thesecond and third semiconductor layers can be applied at half or less thethickness of the first and fourth semiconductor layers to form acombined intermediate layer of approximately the same or less thicknessas that of the first and fourth semiconductor layers with the combinedthickness of the second and third semiconductive layers typically from0.4 to 1.5 mm, or 0.4 to 1 mm. If of a greater thickness than the firstand fourth semiconductor layers, the combined second and thirdsemiconductor layers will have a high conductivity and/or permittivityrelative to these other two semiconductor layers. Each insulation layeris also typically, but not necessarily, of the same thickness as theother insulation layer(s).

Another advantage of the process of this invention is that the innerpower cable can either be immediately processed to a high voltage cable,i.e., passed through the triple extrusion line again, or it can beinventoried, e.g., for one or more days, for later processing, either onthe same or different line or, for that matter, at a different locationaltogether.

In one embodiment the insulation layer is free-radical (e.g., peroxide)cured and thus subjected to continuous vulcanization as described inFIGS. 1 and 3 to thermally activate the crosslinking agent. In oneembodiment the insulation is moisture-cured, and the continuousvulcanization tube described in FIGS. 1 and 3 is replaced with a saunaor similar treatment (not shown) to promote water diffusion into theinsulation layer, or simply allowed to cure under ambient conditions.For moisture-cure compositions, one or more of the compositioncomponents typically contains silane or similar functionality.

Although the invention has been described with certain detail, thisdetail is for the primary purpose of illustration. Many variations andmodifications can be made by one skilled in the art without departingfrom the spirit and scope of the invention as described in the followingclaims.

I claim:
 1. A method of manufacturing a power cable comprising aconductor, semiconductor layers and insulation layers, the processcomprising the steps of: (A) Extruding about the conductor a firstinsulation layer positioned between first and second semiconductorlayers to make an inner power cable comprising: (1) The conductor whichis in contact with, (2) A first semiconductor layer which is also incontact with, (3) A first insulation layer which is also in contactwith, (4) A second semiconductor layer, and (B) Extruding about theinner power cable a second insulation layer positioned between third andfourth semiconductor layers to make the power cable with a totalinsulation thickness of greater than or equal to 9 mm, the power cablecomprising the inner power cable of which the second semiconductor layeris in contact with: (5) The third semiconductor layer which is also incontact with, (6) The second insulation layer which is also in contactwith, (7) The fourth semiconductor layer.
 2. The method of claim 1 inwhich the first insulation layer of the inner power cable is subjectedto free radical promoted crosslinking prior to the extrusion of thesecond insulation layer about the inner power cable.
 3. The method ofclaim 1 in which the first insulation layer of the inner power cable issubjected to moisture-promoted crosslinking prior to the extrusion ofthe second insulation layer about the inner power cable.
 4. The methodof claim 1 in which one or more days elapse between the manufacture ofthe inner power cable and the extrusion of the second insulation layerabout the inner power cable.